diff --git a/include/libimp/byte.h b/include/libimp/byte.h
index aa0c405..5bf2b6c 100644
--- a/include/libimp/byte.h
+++ b/include/libimp/byte.h
@@ -10,10 +10,10 @@
 #include <cstdint>
 #include <cstddef>  // std::byte (since C++17)
 
-#include "fmt/format.h"
-
 #include "libimp/def.h"
 #include "libimp/detect_plat.h"
+#include "libimp/span.h"
+#include "libimp/fmt.h"
 
 #if defined(LIBIMP_CPP_17) && defined(__cpp_lib_byte)
 #define LIBIMP_CPP_LIB_BYTE_
@@ -46,7 +46,7 @@ class byte {
 public:
   constexpr byte() noexcept
     : byte(0) {}
-  
+
   template <typename T, typename = detail::is_integral<T>>
   constexpr byte(T v) noexcept
     : bits_(static_cast<std::uint8_t>(v)) {}
@@ -155,15 +155,27 @@ U *byte_cast(byte const *p) noexcept {
   return reinterpret_cast<U *>(p);
 }
 
-LIBIMP_NAMESPACE_END_
+/// @brief Converts a span into a view of its underlying bytes.
+/// @see https://en.cppreference.com/w/cpp/container/span/as_bytes
 
-template <>
-struct fmt::formatter<::LIBIMP::byte> {
-  constexpr auto parse(format_parse_context& ctx) const {
-    return ctx.end();
-  }
-  template <typename FormatContext>
-  auto format(::LIBIMP::byte b, FormatContext &ctx) {
-    return format_to(ctx.out(), "{:#04x}", static_cast<std::uint8_t>(b));
-  }
-};
+template <typename T, 
+          typename Byte = typename std::conditional<std::is_const<T>::value, byte const, byte>::type>
+auto as_bytes(span<T> s) noexcept -> span<Byte> {
+  return {byte_cast(s.data()), s.size_bytes()};
+}
+
+/// @brief Custom defined fmt_to_string method for imp::fmt
+namespace detail {
+
+inline std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), ::LIBIMP::byte b) {
+  return ::LIBIMP::to_string(static_cast<std::uint8_t>(b), "02x");
+}
+
+template <typename T, 
+          typename = std::enable_if_t<std::is_same<std::decay_t<T>, ::LIBIMP::byte>::value>>
+std::string tag_invoke(fmt_to_string_t, fmt_ref<T> arg) noexcept {
+  return ::LIBIMP::to_string(static_cast<std::uint8_t>(arg.param), arg.fstr);
+}
+
+} // namespace detail
+LIBIMP_NAMESPACE_END_
diff --git a/include/libimp/fmt.h b/include/libimp/fmt.h
index 6e5dd9d..b76f715 100644
--- a/include/libimp/fmt.h
+++ b/include/libimp/fmt.h
@@ -14,10 +14,10 @@
 #include <ctime>    // std::tm
 
 #include "libimp/def.h"
+#include "libimp/fmt_cpo.h"
 #include "libimp/span.h"
 #include "libimp/detect_plat.h"
 #include "libimp/export.h"
-#include "libimp/generic.h"
 
 LIBIMP_NAMESPACE_BEG_
 
@@ -35,19 +35,6 @@ auto spec(char const (&fstr)[N]) noexcept {
   };
 }
 
-namespace detail {
-
-struct fmt_to_string_t {
-  template <typename T>
-  std::string operator()(T &&arg) const {
-    return ::LIBIMP::tag_invoke(fmt_to_string_t{}, std::forward<T>(arg));
-  }
-};
-
-} // namespace detail
-
-constexpr detail::fmt_to_string_t fmt_to_string {};
-
 template <typename... A>
 std::string fmt(A &&...args) {
   std::string joined;
diff --git a/include/libimp/fmt_cpo.h b/include/libimp/fmt_cpo.h
new file mode 100644
index 0000000..dd5bb28
--- /dev/null
+++ b/include/libimp/fmt_cpo.h
@@ -0,0 +1,32 @@
+/**
+ * @file libimp/fmt_cpo.h
+ * @author mutouyun (orz@orzz.org)
+ * @brief String formatting CPO.
+ * @date 2022-11-28
+ */
+#pragma once
+
+#include <string>
+
+#include "libimp/def.h"
+#include "libimp/generic.h"
+
+LIBIMP_NAMESPACE_BEG_
+
+/**
+ * @brief Supports custom fmt_to_string methods for imp::fmt.
+ */
+namespace detail {
+
+struct fmt_to_string_t {
+  template <typename T>
+  std::string operator()(T &&arg) const {
+    return ::LIBIMP::tag_invoke(fmt_to_string_t{}, std::forward<T>(arg));
+  }
+};
+
+} // namespace detail
+
+constexpr detail::fmt_to_string_t fmt_to_string {};
+
+LIBIMP_NAMESPACE_END_
diff --git a/include/libimp/result.h b/include/libimp/result.h
index 8603d62..158a519 100644
--- a/include/libimp/result.h
+++ b/include/libimp/result.h
@@ -10,8 +10,6 @@
 #include <string>
 #include <cstdint>
 
-#include "fmt/format.h"
-
 #include "libimp/def.h"
 #include "libimp/detect_plat.h"
 #include "libimp/export.h"
@@ -75,10 +73,6 @@ struct default_traits<T, std::enable_if_t<std::is_integral<T>::value>> {
   static std::string format(result<T> const &r) noexcept {
     return fmt(*r);
   }
-  template <typename Out>
-  static auto format(result<T> const &r, Out &&out) noexcept {
-    return format_to(out, format(r));
-  }
 };
 
 template <typename T>
@@ -97,10 +91,6 @@ struct default_traits<T, std::enable_if_t<std::is_pointer<T>::value>> {
     }
     return fmt(static_cast<void *>(*r), ", code = ", r.code_value());
   }
-  template <typename Out>
-  static auto format(result<T> const &r, Out &&out) noexcept {
-    return format_to(out, format(r));
-  }
 };
 
 } // namespace detail_result
@@ -168,35 +158,14 @@ public:
 /// @brief Custom defined fmt_to_string method for imp::fmt
 namespace detail {
 
-inline std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), result_code r) noexcept {
+inline std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), result_code r) {
   return fmt("[", (r ? "succ" : "fail"), ", value = ", *r, "]");
 }
 
 template <typename T, typename D>
-std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), result<T, D> r) noexcept {
+std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), result<T, D> r) {
   return fmt("[", (r ? "succ" : "fail"), ", value = ", result<T, D>::default_traits_t::format(r), "]");
 }
 
 } // namespace detail
 LIBIMP_NAMESPACE_END_
-
-template <typename T, typename D>
-struct fmt::formatter<::LIBIMP::result<T, D>> {
-  constexpr auto parse(format_parse_context& ctx) const {
-    return ctx.end();
-  }
-  template <typename FormatContext>
-  auto format(::LIBIMP::result<T, D> r, FormatContext &ctx) {
-    return format_to(::LIBIMP::result<T, D>::default_traits_t::format(r, 
-           format_to(ctx.out(), "[{}, value = ", r ? "succ" : "fail")), "]");
-  }
-};
-
-template <>
-struct fmt::formatter<::LIBIMP::result_code>
-          : formatter<::LIBIMP::result<::LIBIMP::result_type>> {
-  template <typename FormatContext>
-  auto format(::LIBIMP::result_code r, FormatContext &ctx) {
-    return format_to(ctx.out(), "[{}, value = {}]", (r ? "succ" : "fail"), *r);
-  }
-};
diff --git a/include/libimp/span.h b/include/libimp/span.h
index 1e61326..ec88c37 100644
--- a/include/libimp/span.h
+++ b/include/libimp/span.h
@@ -17,11 +17,9 @@
 #include <cstdint>
 #include <initializer_list>
 
-#include "fmt/format.h"
-
 #include "libimp/def.h"
 #include "libimp/detect_plat.h"
-#include "libimp/byte.h"
+#include "libimp/fmt_cpo.h"
 
 #if defined(LIBIMP_CPP_20) && defined(__cpp_lib_span)
 #include <span>
@@ -222,15 +220,6 @@ bool operator==(span<T> a, span<U> b) noexcept {
   return true;
 }
 
-/// @brief Converts a span into a view of its underlying bytes.
-/// @see https://en.cppreference.com/w/cpp/container/span/as_bytes
-
-template <typename T, 
-          typename Byte = typename std::conditional<std::is_const<T>::value, byte const, byte>::type>
-auto as_bytes(span<T> s) noexcept -> span<Byte> {
-  return {byte_cast(s.data()), s.size_bytes()};
-}
-
 /// @brief Constructs an object of type T and wraps it in a span.
 /// Before C++17, template argument deduction for class templates was not supported.
 /// @see https://en.cppreference.com/w/cpp/language/template_argument_deduction
@@ -278,22 +267,20 @@ inline auto make_span(std::string const &str) noexcept -> span<char const> {
   return {str.data(), str.size()};
 }
 
-LIBIMP_NAMESPACE_END_
+/// @brief Custom defined fmt_to_string method for imp::fmt
+namespace detail {
 
 template <typename T>
-struct fmt::formatter<::LIBIMP::span<T>> {
-  constexpr auto parse(format_parse_context& ctx) const {
-    return ctx.end();
+std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), span<T> s) {
+  if (s.empty()) {
+    return {};
   }
-  template <typename FormatContext>
-  auto format(::LIBIMP::span<T> s, FormatContext &ctx) {
-    if (s.empty()) {
-      return format_to(ctx.out(), "");
-    }
-    auto appender = format_to(ctx.out(), "{}", s[0]);
-    for (std::size_t i = 1; i < s.size(); ++i) {
-      appender = format_to(appender, " {}", s[i]);
-    }
-    return appender;
+  auto appender = fmt(s[0]);
+  for (std::size_t i = 1; i < s.size(); ++i) {
+    appender += fmt(' ', s[i]);
   }
-};
+  return appender;
+}
+
+} // namespace detail
+LIBIMP_NAMESPACE_END_
diff --git a/include/libimp/system.h b/include/libimp/system.h
index ce207c6..69ab783 100644
--- a/include/libimp/system.h
+++ b/include/libimp/system.h
@@ -13,7 +13,7 @@
 #include "libimp/def.h"
 #include "libimp/export.h"
 #include "libimp/result.h"
-#include "libimp/fmt.h"
+#include "libimp/fmt_cpo.h"
 
 LIBIMP_NAMESPACE_BEG_
 namespace sys {
@@ -73,12 +73,3 @@ inline std::string tag_invoke(decltype(::LIBIMP::fmt_to_string), error r) noexce
 
 } // namespace sys
 LIBIMP_NAMESPACE_END_
-
-template <>
-struct fmt::formatter<::LIBIMP::sys::error>
-          : formatter<std::string> {
-  template <typename FormatContext>
-  auto format(::LIBIMP::sys::error r, FormatContext &ctx) {
-    return format_to(ctx.out(), ::LIBIMP::sys::error_msg(r.code()));
-  }
-};
diff --git a/src/libimp/CMakeLists.txt b/src/libimp/CMakeLists.txt
index f21db72..bcd46f0 100644
--- a/src/libimp/CMakeLists.txt
+++ b/src/libimp/CMakeLists.txt
@@ -29,8 +29,6 @@ target_include_directories(${PROJECT_NAME}
   PUBLIC  ${LIBIPC_PROJECT_DIR}/include
   PRIVATE ${LIBIPC_PROJECT_DIR}/src)
 
-target_link_libraries(${PROJECT_NAME} PUBLIC fmt)
-
 install(
   TARGETS ${PROJECT_NAME}
   RUNTIME DESTINATION bin
diff --git a/test/imp/test_imp_byte.cpp b/test/imp/test_imp_byte.cpp
index ad2fd37..b8f9e26 100644
--- a/test/imp/test_imp_byte.cpp
+++ b/test/imp/test_imp_byte.cpp
@@ -3,6 +3,7 @@
 
 #include "libimp/byte.h"
 #include "libimp/span.h"
+#include "libimp/fmt.h"
 
 TEST(byte, construct) {
   {
@@ -38,13 +39,13 @@ TEST(byte, compare) {
 TEST(byte, fmt) {
   {
     imp::byte b1, b2(31);
-    EXPECT_EQ(fmt::format("{}", b1), "0x00");
-    EXPECT_EQ(fmt::format("{}", b2), "0x1f");
+    EXPECT_EQ(imp::fmt(b1), "00");
+    EXPECT_EQ(imp::fmt(b2), "1f");
+    EXPECT_EQ(imp::fmt(imp::spec("03X")(b2)), "01F");
   }
   {
     imp::byte bs[] {31, 32, 33, 34, 35, 36, 37, 38};
-    EXPECT_EQ(fmt::format("{}", imp::make_span(bs)), 
-              "0x1f 0x20 0x21 0x22 0x23 0x24 0x25 0x26");
+    EXPECT_EQ(imp::fmt(imp::make_span(bs)), "1f 20 21 22 23 24 25 26");
   }
 }
 
diff --git a/test/imp/test_imp_result.cpp b/test/imp/test_imp_result.cpp
index e79600f..162264c 100644
--- a/test/imp/test_imp_result.cpp
+++ b/test/imp/test_imp_result.cpp
@@ -2,9 +2,9 @@
 #include <sstream>
 
 #include "gtest/gtest.h"
-#include "fmt/format.h"
 
 #include "libimp/result.h"
+#include "libimp/fmt.h"
 
 TEST(result, ok) {
   imp::result_code ret;
@@ -59,23 +59,23 @@ TEST(result, compare) {
 TEST(result, fmt) {
   {
     imp::result_code r1;
-    EXPECT_EQ(fmt::format("{}", r1), "[fail, value = 0]");
+    EXPECT_EQ(imp::fmt(r1), "[fail, value = 0]");
     imp::result_code r2(true, 65537);
-    EXPECT_EQ(fmt::format("{}", r2), "[succ, value = 65537]");
+    EXPECT_EQ(imp::fmt(r2), "[succ, value = 65537]");
     imp::result_code r3(0);
-    EXPECT_EQ(fmt::format("{}", r3), "[succ, value = 0]");
+    EXPECT_EQ(imp::fmt(r3), "[succ, value = 0]");
   }
   {
     imp::result<int> r1 {false, -123};
-    EXPECT_EQ(fmt::format("{}", r1), fmt::format("[fail, value = {}]", -123));
+    EXPECT_EQ(imp::fmt(r1), imp::fmt("[fail, value = ", -123, "]"));
     imp::result<void *> r2 {&r1};
-    EXPECT_EQ(fmt::format("{}", r2), imp::fmt("[succ, value = ", (void *)&r1, "]"));
+    EXPECT_EQ(imp::fmt(r2), imp::fmt("[succ, value = ", (void *)&r1, "]"));
     int aaa {};
     imp::result<int *> r3 {&aaa};
-    EXPECT_EQ(fmt::format("{}", r3), imp::fmt("[succ, value = ", (void *)&aaa, "]"));
+    EXPECT_EQ(imp::fmt(r3), imp::fmt("[succ, value = ", (void *)&aaa, "]"));
     imp::result<int *> r4 {nullptr};
-    EXPECT_EQ(fmt::format("{}", r4), imp::fmt("[fail, value = ", nullptr, ", code = 0]"));
+    EXPECT_EQ(imp::fmt(r4), imp::fmt("[fail, value = ", nullptr, ", code = 0]"));
     r4 = {nullptr, 1234};
-    EXPECT_EQ(fmt::format("{}", r4), imp::fmt("[fail, value = ", nullptr, ", code = 1234]"));
+    EXPECT_EQ(imp::fmt(r4), imp::fmt("[fail, value = ", nullptr, ", code = 1234]"));
   }
 }
\ No newline at end of file
diff --git a/test/imp/test_imp_span.cpp b/test/imp/test_imp_span.cpp
index f49ae25..630bca2 100644
--- a/test/imp/test_imp_span.cpp
+++ b/test/imp/test_imp_span.cpp
@@ -6,6 +6,8 @@
 
 #include "libimp/span.h"
 #include "libimp/countof.h"
+#include "libimp/byte.h"
+#include "libimp/fmt.h"
 
 TEST(span, to_address) {
   int *a = new int;
@@ -56,6 +58,6 @@ TEST(span, span) {
 }
 
 TEST(span, fmt) {
-  EXPECT_EQ(fmt::format("{}", imp::span<int>{}), "");
-  EXPECT_EQ(fmt::format("{}", imp::make_span({1, 3, 2, 4, 5, 6, 7})), "1 3 2 4 5 6 7");
+  EXPECT_EQ(imp::fmt(imp::span<int>{}), "");
+  EXPECT_EQ(imp::fmt(imp::make_span({1, 3, 2, 4, 5, 6, 7})), "1 3 2 4 5 6 7");
 }
\ No newline at end of file
diff --git a/test/imp/test_imp_system.cpp b/test/imp/test_imp_system.cpp
index d4128ed..b3d46c2 100644
--- a/test/imp/test_imp_system.cpp
+++ b/test/imp/test_imp_system.cpp
@@ -3,11 +3,11 @@
 #include <string>
 
 #include "gtest/gtest.h"
-#include "fmt/format.h"
 
 #include "libimp/system.h"
 #include "libimp/detect_plat.h"
 #include "libimp/codecvt.h"
+#include "libimp/fmt.h"
 
 #if defined(LIBIMP_OS_WIN)
 #include <Windows.h>
@@ -25,11 +25,11 @@ TEST(system, error_code) {
 
   imp::sys::error e_obj {err};
   EXPECT_EQ(err.value(), e_obj.value());
-  auto e_msg = fmt::format("{}", imp::sys::error_msg(imp::sys::error_code()));
+  auto e_msg = imp::fmt(imp::sys::error_msg(imp::sys::error_code()));
   std::stringstream ss;
   ss << imp::sys::error{};
   EXPECT_EQ(e_msg, ss.str());
-  EXPECT_EQ(e_msg, fmt::format("{}", imp::sys::error()));
+  EXPECT_EQ(e_msg, imp::fmt(imp::sys::error()));
   std::cout << e_msg << "\n";
 }